1. Field of the Invention
This invention relates to implants and, more particularly, to an implant for fixation of first and second bones, as to allow fusion therebetween. The invention is also directed to a method of fixing first and second bones using the implant.
2. Background Art
It is known to fuse the talus and the calcaneus (the “heel bone”) to address certain foot conditions. The talus sits on top of the calcaneus. Superiorly (on its upper surface), the talus forms a major part of the ankle joint, this joint providing the motion that allows the foot to go up and down. Inferiorly (on its lower surface), the talus forms a separate joint with the calcaneus, the subtalar joint. The subtalar joint is actually not a single joint surface but rather formed by three separate and distinct facets, with the posterior facet being the major part and the anterior and middle facets small contributions to this joint. The three joints work together, however, and provide the motion that allows the heel to invert or evert from side to side so that the foot can maintain a flat contact with uneven ground.
On the lateral side of the hindfoot between the posterior facet and the other facets of the subtalar joint (middle, anterior), there is a conical shaped interval between the talus and the calcaneus. This cone of soft tissue between the two bones is called the sinus tarsi. It is wider laterally and tapers medially as it crosses between the two bones.
Certain conditions such as fractures of the calcaneus or flat feet can lead to arthritis of the subtalar joint, which can cause considerable pain and dysfunction with walking. When this is not controlled with simple methods of treatment, such as anti-inflammatory medication or shoe inserts, a preferred treatment can be to perform arthrodesis, or fuse, the subtalar joint. Although arthrodesis creates a single bone between the talus and calcaneus and eliminates all motion between the two bones, it can be very effective for controlling pain.
The general principles of arthrodesis or fusing a joint are no different when addressing the subtalar joint as compared to any other joint. The surface cartilage is removed and the bone roughened on the joint surfaces to create a surface that is conducive to causing the bones on either side to heal to one another. Frequently, this includes the application of bone graft, or porous cancellous bone that is removed and applied across the surfaces of the joint. In a simplistic way of looking at this, the surgeon is trying to get the bone on each side of the joint to ‘think’ it has been fractured so that they heal to one another.
In addition to preparation of the joint surface and application of bone graft, it is important to stabilize the joint to eliminate micromotion between the two bones while healing takes place. This helps to prevent disruption of the microscopic crossing channels of bone that are laid down during the healing phase, a process that prevents the two bones from healing to one another.
There are different methods that are currently used to rigidly fix the two bones in order to promote arthrodesis of the subtalar joint.
One method is to apply a plate across the surface of the two bones. Plate fixation has the disadvantage of requiring a large incision to allow application of a plate large enough to provide fixation. The skin in this area is frequently thin and has a poor vascular supply, and large incisions are prone to break-down, dehiscence, and infection. In addition, since plates are applied to the surface of the bone, they need to be thick enough to withstand the relatively large loads from bending torque; this bulk can contribute to skin breakdown or irritation.
Another method is to place screws across the joint. Simple screw fixation, however, is not a strong method of fixation and may fail with the loads that are applied with weight bearing.
Still another method of fixation is to place staples across the joint. Since staples are on the surface of the bone they are subject to bending torque and can loosen. In addition, staples only fix a single surface of the joint which limits fixation across the joint surface.
All current methods of fixation are deficient in one or more respects, as described above. Many of the methods apply implants to the surfaces of the bone which are at significant distances from the joints being fused; these are less effective at controlling micromotion across the joint surfaces and are subject to considerable cantilever bending loads. In addition, these methods require direct tedious intraarticular debridement of the joint surface with extensive exposures in order to expose raw bone surfaces on either side of the joint.
Those in the medical art continue to seek out better methods of fixation of bones, particularly at the subtalar joint.